The present invention relates to a process for the preparation of optically active menthol from geraniol or nerol or mixtures of geraniol and nerol.
Menthol is one of the most important aroma chemicals, the bulk of which is still isolated from natural sources. For totally synthetic routes to menthol, especially the naturally occurring enantiomer L-menthol, on the industrial scale, there is consequently a constant need to optimize the economics of the process. In particular, the synthesis of L-menthol from inexpensive achiral educts is therefore still a challenge.
L-menthol can be synthesized by pursuing two strategies: On the one hand, racemic menthol, obtainable e.g. by the hydrogenation of thymol, can be obtained after esterification by racemate resolution (by crystallization or enzymatic resolution), as described e.g. in EP-A 0 743 295, EP-A 0 563 611 or EP-A 1 223 223. On the other hand, it is also possible to pursue asymmetric synthesis strategies comprising an enantioselective synthesis step.